The invention relates to optical modulators, and in particular, to a compound cavity reflection modulation laser system providing frequency modulation.
Fiber optics communication systems require compact light emitting sources capable of generating single mode, narrow linewidth radiation in the 1.3-1.6 xcexcm wavelength range. Some of the existing semiconductor lasers, for example, InGaAsP DFB lasers, can meet requirements for high power and proper wavelength operation. However, the ever increasing demand for network bandwidths, requiring narrower channel spacing in dense wavelength division multiplexing (DWDM) optical networks, poses a serious technological challenge on modulation schemes used in optical communication systems.
Nowadays there are two major types of modulation techniques which are widely used in the optical communication industry: a direct amplitude modulation of semiconductor lasers, and an external amplitude modulation. In the direct modulation, the electrical signal drives the laser from the level which is slightly above the threshold current to the level which is well above the threshold current to obtain direct intensity modulation. In this type of modulation, the frequency chirp is the dominant factor which determines the system performance and limits its practical applications.
In contrast, the external modulation can suppress the frequency chirp to a large extent. However, since this type of modulation requires a modulator to be placed in front of the laser, it results in substantial absorption of the output laser power within the modulator. The absorbed power generates heat that impacts the reliability of the external modulator and/or results in the frequency chirp of the laser when the laser and the modulator are integrated together. It is also either bulky or expensive to isolate the two components or to integrate them together.
Another well known modulation technique, the frequency modulation (FM) technique, is known to give much better receiver sensitivity due to its immunity to the noise added into the transmitting channel in various stages of communication process. In the optical domain, FM schemes can be implemented in different ways, none of them unfortunately having found wide practical applications. The main reasons, limiting performance of FM, are associated with a limited speed of modulation determined by the relaxation frequency of lasers and a significant amount of thermal chirp resulted from the relatively small current modulation.
There is another technique for providing optical modulation known as a reflection modulation technique. In this configuration, an output light of a semiconductor laser is sent to a piece of passive waveguide whose facets are anti-reflection coated or left as cleaved. The laser is fully isolated from the waveguide modulator by means of an optical circulator. By modulating the passive waveguide, the round trip phase inside the passive waveguide section is changed, and the entire waveguide serves as a Fabry-Perot (FP) etalon providing either transmission or reflection. Thus, the semiconductor laser is totally isolated from the entire modulation process and therefore is not affected by the external modulation operation. The resulting signal is directly amplitude modulated (AM) only. The problems associated with this scheme are similar to that of other types of external AM modulation. It is bulky in size because of the requirement to have the optical isolator or circulator, and since the reflection of the beam may partially come back into the laser, it needs to maintain an excellent isolation and beam redirection during operation.
Accordingly, there is a need in industry for the development of new schemes for optical modulation which would eliminate frequency chirps associated with known modulation schemes while maximizing the output optical power.
It is therefore an object of the present invention to provide a system for modulating laser light which would avoid the afore-mentioned problems.
Thus, according to one aspect of the present invention there is provided a compound cavity reflection modulation laser system, comprising:
a single wavelength laser, having a first cavity defined by a front laser facet and a rear laser facet;
a phase modulation element having a second cavity defined by a front element facet and a rear element facet, the first and second cavities forming a compound cavity of the laser system defined by the front laser facet and the rear element facet;
the phase modulation element receiving light generated in the first cavity and providing phase modulation of said light to produce phase modulated light, the phase modulated light being fed back into the first cavity so as to provide interference of light formed in the first and second cavities, the interference effects resulting in complex reflection modulation of the rear laser facet, the reflection modulation providing frequency modulation of the output light.
Beneficially, the front element facet is attached to the rear laser facet so that the first and second cavities are aligned along the same line. The phase modulation element is a passive waveguide which is preferably a reverse biased semiconductor waveguide comprising a multiple quantum well region. The laser and the modulation element are hybrid integrated into the system, with the monolithic integration of the laser and the modulation element on the same chip being an alternative design. Advantageously, the laser is a single mode semiconductor DFB laser comprising a multiple quantum well structure. Preferably the DFB laser is a gain-coupled laser or loss-coupled laser, conveniently comprising a complex coupled grating formed by etching grooves directly through the multiple quantum well structure. Alternatively, the laser may be any other known single wavelength semiconductor laser generating light within the required range of wavelengths.
Conveniently, each of the element facets is one of the cleaved facet and high reflection coated facet. Alternatively, the front element facet and the rear laser facet may be either anti-reflection coated or left as cleaved.
Optionally, the system may further comprise means for frequency modulation (FM) into intensity modulation (IM) conversion, e.g. semiconductor or fiber Mach-Zehnder interferometer, or a band pass filter.
According to another aspect of the invention, there is provided a reflection modulation compound cavity laser system, comprising:
a single mode DFB semiconductor laser, having a first cavity defined by a front laser facet and a rear laser facet;
a phase modulation element, the element being a passive waveguide having a second cavity defined by a front element facet and a rear element facet, the first and second cavities forming a compound cavity for the laser system defined by the front laser facet and the rear element facet so that the rear laser facet and the front element facet are attached to each other and the first and second cavities are aligned along same axis;
the phase modulation element receiving light generated in the first cavity and modulating said light in the second cavity to produce phase modulated light, the phase modulated light being fed back into the first cavity so as to provide modulation of the effective complex reflection of the rear laser facet due to interference of light from the first and second cavities, the reflection modulation resulting in frequency modulation of the output light.
The reflection modulation system described above offers many unique features over other known modulation schemes by providing much higher output power, eliminating thermal and other types of frequency chirps, requiring lower drive voltage in the modulation element, providing higher device yield in large scale production and being suitable for high speed and long haul data transmission.